This invention relates to a film with a skin layer of a thermoplastic polymer with an additive to promote metal adhesion to the surface of this layer. This additive is one or more compounds of the formula, R-X, where R is a non-polar aliphatic hydrocarbyl group having an average of from 20 to 200 carbon atoms and X is a polar group, such as xe2x80x94COOH or xe2x80x94CH2OH.
Plastic materials such as polymeric films have been widely used for packaging various food and non-food products. In order to ensure proper preservation of products packaged in such polymeric films, it is necessary to provide the films with barriers against transmission of air, moisture, deleterious flavors, etc. Unmodified polymeric films, however, typically lack sufficient gas and moisture barrier characteristics needed for proper packaging requirements. For example, polypropylene films are particularly preferred in the manufacture of packaging films due to their low cost and ease of manufacture. Such films, however, inherently permit the transmission of oxygen and water vapor from the outside of the film to the inside of the package made up of the film. As will be recognized by those skilled in the art, transmission of oxygen and water vapor through food packaging promotes deterioration of the foods packaged therein.
Multi layer polymeric films have been developed having improved gas and moisture barrier characteristics. In particular, it is known that films incorporating poly-vinyl alcohol (PVOH) or ethylene-vinyl alcohol copolymer (EVOH) as a co-extruded or coated outer layer, on which a metal, such as aluminum, is vapor deposited, display outstanding oxygen and moisture barrier properties.
An example of a co-extruded metallizable film with an EVOH skin is described in U.S. Pat. No. 5,153,074. In particular, an EVOH layer is applied to a surface of a polymer substrate layer, which includes a maleic acid anhydride modified propylene homopolymer.
U.S. Pat. No. 5,192,620 discloses an oriented polypropylene base layer having a coating on one surface of a blend of a vinyl alcohol homopolymer or copolymer and an ethylene-acrylic acid copolymer, with a further metal layer thereon. Films incorporating ethylene-vinyl alcohol as co-extruded or coated layers, however, are difficult and expensive to manufacture.
U.S. Pat. No. 5,194,318, the disclosure of which is incorporated herein by reference, discloses a metallized oriented thermoplastic film combination having a propylene homopolymer or copolymer substrate with a high density polyethylene skin layer on at least one side of the substrate. The high density polyethylene skin layer is further covered by a thin metal layer which has been vapor deposited thereon. In one embodiment, the high density polyethylene skin layer is flame or corona discharge treated, and the metal coating is thereafter deposited onto the flame or corona discharge treated layer. Although films which incorporate a high density polyethylene skin do not suffer from the deleterious effects of metal pick off, such films do not provide oxygen and water vapor barrier properties as good as those of films with EVOH or PVOH skins.
U.S. Pat. No. 5,981,079 describes a multi layer film having enhanced barrier properties against transmission of oxygen and water vapor. The multi layer film includes a polypropylene base layer, with a high density polyethylene layer on at least one surface of the polypropylene base layer. The polyethylene layer includes a surface which has been subjected to plasma treatment with a hydroxyl-donating material such as a methanol. The film further includes a metal layer deposited on the plasma treated surface, such as a layer of vacuum deposited aluminum.
A need exists for a packaging film which is simple and inexpensive to manufacture and which is capable of providing enhanced barrier properties to oxygen and water vapor transmission at low manufacturing cost.
There is provided a multi layer film comprising:
(a) a core layer comprising a thermoplastic polymer; and
(b) at least one metallizable skin layer comprising a non-polar thermoplastic polymer and at least one metal adhesion promoting compound of the formula R-X, where R is a non-polar aliphatic hydrocarbyl group having an average of from20 to 200 carbon atoms and X is a polar group.